International Journal of Applied Science and Technology Vol. 3 No. 5; May 2013 Optical Characterization and Properties of Polymeric Materials for Optoelectronic and Photonic Applications A. N. Alias Z. M. Zabidi A.M.M. Ali M. K. Harun Faculty of Applied Sciences, Universiti Teknologi MARA 40450 Shah Alam, Selangor D.E Malaysia M.Z.A. Yahya Faculty of Science & Defence Technology, Universiti Pertahanan Nasional Malaysia, Kemsg. Besi, 57000 Kuala Lumpur Malaysia Abstract The optical properties of polymers attract considerable attention because of their potential optoelectronic applications, such as in polymer-based light-emitting diodes, light electrochemical cells, and solar cells. To optimize the performance of optoelectronic and photonic devices, some polymers are modified through the introduction of specific end groups, copolymerization, grafting, crosslinking, blending, and hybridization with other inorganic polymers. All these techniques affect the optical properties of the polymer. This paper reviews the fundamental emission and optical properties, such as absorption, reflection, and polarization effect, of numerous polymer materials. These aspects are important to be considered in optoelectronic devices. Introduction Polymers are widely used in electrical and electronic applications. In early works, polymers have been used as insulators because of their high resistivity and dielectric properties. Polymer-based insulators are used in electrical equipment to separate electrical conductors without passing current through themselves. The insulator applications of polymers include printed circuit boards, wire encapsulants, corrosion protective electronic devices, and cable sheathing materials [1]. Polymers have several advantages, such as easy processing, low cost, flexibility, high strength, and good mechanical properties. In the microelectronic fabrication industry, polymers are used in the photolithography process. In 1977, Shirakawa et al. [2], macdiarmid [3], and Heeger[2]found that the conducting polymer polyacetylene can be used as a dopant and change the conductivity of the doped material. Since this discovery, polymers have been extensively studied as new materials for electronic and optoelectronic applications. To enhance the conductivity using polymers as well as to create new optical and electrical properties of polymer, four process of doping have been identified, including chemical doping, photochemical doping, electrochemical doping, and interfacial doping [3, 4]. Optoelectronic and electronic materials are gaining interest because of their implementation of various functions of light, such as modulation, generation, switching, and detection in the optical waveguide structure formed on the substrate [5]. Polymers can become suitable materials for optoelectronic and photonic applications. Much progress has been made in understanding the fundamental physical and chemical properties of optoelectronic and photonic devices to optimize device efficiency. However, the correlation between the physical and chemical properties is not fully understood. One of the most important characteristics is the optical properties. Optical characterizations of polymer such as optical absorption, luminescence spectra, infrared dichroism, and Raman polarization are the most vital tools used to examine the electronic properties of polymers [6-8]. 11 © Center for Promoting Ideas, USA www.ijastnet.com These techniques rely on the transition of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO)or the nonlinear optical properties of polymeric material.To optimize the performance of optoelectronic and photonic devices, polymers are modified through the introduction of specific end groups, copolymerization, grafting, crosslinking, blending, and hybridization with other inorganic polymers. In copolymers, the monomeric unit may be arranged randomly, alternating, blocked, or grafted. Another polymer can be joined with other polymers by crosslinking. Polymer blending is the simplest, most well-known technique in polymer engineering for creating new solid materials with more enhanced properties than homopolymers. Polymer blending with other polymer materials or small molecules such asanthracene, pyrene, or pyridine has been performed [9].Polymer materials have also been mixed with other salts or hybridized with other inorganic/nanoparticle materials to modify the optical and electrical properties of the polymer system. All these techniques affect the optical properties of polymers. This review is aimed to understand the fundamental optical properties of polymer materials widely used in optoelectronic and photonic applications. In section 2, we review the basic device operation and polymeric material which been used in optoelectronic and photonic devices. While in section 3, experimental and physical description to characterize the optical parameter in polymeric material. The optical properties such as optical absorption band, dispersion behavior, anisotropic, excitation and emission spectra have been explained in section 4. The effect of conjugation length in Raman Spectra also had been discussed in same section. By knowing the fundamental optical properties of polymers, we believe that devices operation can be improved and further developments can be made. The studied optical properties of polymer materials include absorption, reflection, and emission properties, as well as basic optoelectronic device operation. Polymeric Material and Basic Operation in Optoelectronic and Photonic Devices Polymer Light-Emitting Diodes (pleds) In 1987, Tang from Kodak reported low-voltage organic light-emitting diodes (leds) [10]. Three years later, Burroughes invented the first pleds using poly (phenylenevinylene) (PPV) [11]. Since then, research on pleds has rapidly grown. Basically, pleds consist of a hole transport layer (HTL) and an electron transport layer (ETL) sandwiched by an anode and a cathode. Adachi proposed that materials for the HTL must have the abilities to facilitate a low driving voltage, block the electron passing through the emission layer which known as electron blocking layer, and prevent quenching molecular exciton [12]. For single HTL/EBL, aromatic amine derivative has widely been used and it shows good thermal stability[13]. Non-conjugated polymers such as polyaniline (PANI) and polycarbazole (PVK)have also been identified to be suitable candidates for HTL/EBL[14, 15]. Currently, deoxyribonucleic acid (DNA) has been used as EBL in PLED application or known as bioled. The luminescence efficiency is higher in LED using DNA as EBL comparedto PVK and poly(methyl methacrylate) (PMMA)[16].DNA used in EBL is normally extracted from salmon spermdna to perform DNA complex. The multilayer bioled made from tris-(8-hydroxyquinoline) aluminum (Alq3) as the emitting layer shows low turn-on voltage (~ 5V), high efficiency, high brightness and also good white color stability[17].Madhwal et al. Reported that salmon DNA as EBL also enhances the performance of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly[9,9-dioctyfuorene] (PFO)- based light emitting diode[18]. They also proposed that EBL made from DNA has high potential barrier which could trap electron injected from cathode. The recombination with hole injected from anode could produce high luminescence intensity.On the other hand, the ETL materials must have excellent ETL and exciton confinement [19]. When voltage is applied to the anode, holes are injected to the HTL and electrons are injected to the ETL, as shown in Figure 1. To ensure that the charge carriers are effectively injected, the anode must have a high work function and fit with the HOMO HTL and vice versa at the cathode [20, 21]. The carriers are injected either through the Fowler–Nordhem tunneling mechanism or Schottky thermionic emission [22]. Electrons and holes migrate apart from the electrode to form excitons at the polymer emitting layer. The emitting layer for confining the exciton is the key factor for color emission in PLED.The suitable material for emitting layer must consist of conjugated π-bonding. 12 International Journal of Applied Science and Technology Vol. 3 No. 5; May 2013 This material can be small molecule oligomer which known as small molecule light emitting diode (SMOLED) or polymer molecule-based PLED. In fabrication process, since small molecule has poor solubility, therefore vacuum vapor deposition technique is the best choice to fabricate SMOLED. While, PLED can be fabricated by solution process namely spin coating or ink-jet printing which are suitable for low cost and wide area display application. Devices-based multilayer structure SMOLED are more complexes and sophisticated architecture compared to the PLED which it depends on the molecular energy gap alignment [23]. There are two types of exciton based on the quantum number, namely, singlet and triplet excitons. Normally, triplet excitons tend to undergo non-radiative recombination, which results in joule heating in pleds [24]. To avoid this phenomenon, organic ligands such bis[2-(2’-benzothienyl)- Pyridinaton,C3’)iridium (acetylacetonate) have been used to transfer the triplet state to the f level of Ir and generate electro phosphorescence[25]. For electro phosphorescence light emitting diode, phosphorescence SMOLED has shown a higher efficiency compared to the phosphorescence PLED[26]. In order to have both advantages from SMOLED and PLED, a hybrid LED has been